Replace old-style raise with six.reraise

The old style raise is invalid syntax in python3. As noted in the docs,
this adds one more frame in the traceback, but I think this is
acceptable:

    <ipython-input-7-bcc5cba3de3f> in <module>()
         16     except:
         17         pass
    ---> 18     six.reraise(*x)

    /usr/lib/python3.6/site-packages/six.py in reraise(tp, value, tb)
        691             if value.__traceback__ is not tb:
        692                 raise value.with_traceback(tb)
    --> 693             raise value
        694         finally:
        695             value = None

    <ipython-input-7-bcc5cba3de3f> in <module>()
          9
         10 try:
    ---> 11     x()
         12 except:
         13     x = sys.exc_info()

Also note that this uses six, which is not formally a dependency yet,
but is included indirectly since most packages depend on it.

Signed-off-by: Adrian Tschira <nota@notafile.com>

synapse/rest/media/v1/media_storage.py

@@ -16,6 +16,8 @@
 from twisted.internet import defer, threads
 from twisted.protocols.basic import FileSender
 
+import six
+
 from ._base import Responder
 
 from synapse.util.file_consumer import BackgroundFileConsumer
@@ -119,7 +121,7 @@ class MediaStorage(object):
                 os.remove(fname)
             except Exception:
                 pass
-            raise t, v, tb
+            six.reraise(t, v, tb)
 
         if not finished_called:
             raise Exception("Finished callback not called")